1. Field of the Invention
The present invention relates to estimation of occlusion in relation to hearing aid use. The invention more particularly relates to a system for estimating the occlusion effect comprising a hearing aid adapted for being set up for operation in an occlusion measurement mode.
When a hearing aid is placed in the ear of a user with an at least partially acoustically sealing ear mould it occludes the ear canal. Speech produced by the user generates not only sound pressure in the air but also vibrations in the wall of the ear canal. In the occluded ear this causes an elevation of the sound level of the user's own voice at the eardrum, especially in the lower frequencies. For many hearing aid users their own voice then sounds hollow or boomy, and this is known as the Occlusion Effect (OE). The OE can be perceived so annoying to the user, that it becomes a major obstacle in the hearing aid use.
When fitting a hearing aid it is in many cases convenient to be able to measure the OE in order to be able to reduce the OE as much as possible, considering that a sufficient amplification still has to be obtained. Possibilities for reducing OE are typically to increase the diameter of a ventilation channel in the ear plug or in the hearing aid.
The OE of a speaker's own voice is the ratio between the sound pressures generated at the eardrum by the voice when the ear canal is occluded by an ear mould and the hearing aid electrical amplification is switched off, and the sound pressures generated at the eardrum by the voice when the ear canal is open. I.e. OE=pdrum, occluded/pdrum, open.
Both of the quantities pdrum, occluded and pdrum, open are functions of time and frequency. So the OE is a time variant frequency dependent transfer function. To find the OE the two sound pressures (pdrum, occluded and pdrum, open) must be determined with sufficient accuracy and resolution in both time and frequency. If pdrum, open is zero the OE is not defined.
2. The Prior Art
In PCT/EP2009/050759, published as WO-A1-2010083888, it is explained how the OE can be found from a slightly different set of sound pressures (pdrum,occluded and pext,occluded) and a correction term (that can be regarded constant for a given set-up). For the lower frequency range in which the OE is most significant, the estimation of the Occlusion Effect by OE=k·pdrum, occluded/pext, occluded, (where k is a constant correction term) is relatively good. If pext,occluded is zero the OE is not defined.
WO-A1-2008/017326 describes occlusion effect measurement by using the hearing aid, relying on the users own voice as a sound source.
To make a practical measurement of the OE, a measurement of the two sound pressures, pdrum, occluded and pext, occluded, must be made. If the measurement of one or both sound pressures does not provide a sufficiently accurate result the OE value will not be valid and should be discarded. Different limitations in the procedure of the measurement of the sound pressures may in certain situations lead to inaccurate results. In order to discard those cases it is important to detect them properly.
As discussed in PCT/EP2009/050759 each of the two sound pressures can be captured by pressure sensitive transducers (such as a hearing aid microphone or a hearing aid receiver coupled to be used as a microphone). The output of the transducers can be analyzed in frequency bands by a filter bank (a bank of adjacent band pass filters), and the strength of the signal in each band be detected. The general assumption is that the signal strength in each band represents the sound pressure in that particular frequency band.
When measuring OE by using the hearing aid, the filter bank of the hearing aid, or an equivalent filter bank, is usually applied for splitting the obtained values of pdrum, occluded and pext, occluded into respective filter bands.
As also discussed in PCT/EP2009/050759 such frequency analysis can be compromised due to the finite, limited selectivity of the band pass filters. This is especially critical if the signal truly inside the frequency range of a band is low and the signal in a nearby band is higher. Due to the limited selectivity some of the higher signal power of the nearby band will be detected together with the low signal power inside the band. This means that a signal at one frequency which ideally should only be detected in one corresponding frequency band will also be detected in several neighboring frequency bands. This is here called leakage and may lead to erroneous results if measured values of the sound pressure severely contaminated with leakage are used for calculating the OE. The leakage may also be referred to as smearing or spectral splatter.
When the hearing aid users own voice is the sound source, the values of pdrum, occluded and pext, occluded are also expected to have broad and rather smooth frequency distributions on a long term basis (signal power spread across the entire speech spectrum). However, on a short term basis the power may be concentrated in narrow frequency bands (corresponding to the fundamental frequency and its harmonics during voiced speech). It is in this case not directly possible to see from the obtained values in the frequency bands which part of the signal is caused by an actual sound pressure level within the particular bands and which part is caused by leakage from other bands.